Fixed?

I used to commute to work in Portland on an old road bike I'd converted to single-speed. My condo was in the city less than a mile from work, but on a steep hill, so it was set up with a 42x18, just low enough to (barely) get up the hill, yet still high enough to cruise around town, and the freewheel allowed me to coast down the hill. I'm now about 18 miles from work, but I'm thinking of going fixed and combining some winter fixed gear training with commuting. I'm thinking 42x16 since it's flat here. Some possible issues: the hub is an old freewheel hub, so I'm not sure if it can take a track cog, since I think they need a separate set of threads for a lock ring. Although I had a local framebuilder braze on some cantilever studs, there's still almost no fender clearance since it's an old school racing bike - I suppose I can use the expensive Race Blades (sp?) though. Probably the biggest issue is the frame itself. Fujis are notorious for short top tubes. In this case, although I normally ride a 56cm frame, the Fuji is supposed to be a 58cm, but has a shorter top tube than normal 56cm frames, plus shorter cranks. It also has a classic quill stem, but I'm pretty sure I can still find those. Nevertheless, my riding position would be noticeably different than my other bikes which I've taken pains to set up identically. I'm wondering if putting in hours on an ill-fitting fixed gear bike would mess me up. I could always move everything over to a Steamroller frameset, but that would be expensive, and besides, the Fuji was my first serious road bike and I'm kind of attached to it. Any of you fixies out there have any input?